1. Field of the Invention
The present invention relates to radiotelephony, and more particularly, it relates to a system for preventing stealing of the line in radiotelephony in which the master telephone set cannot be enabled by any subsidiary telephone set other than a specifically designated subsidiary telephone set.
2. Description of the Prior Art
In radiotelephony in which communication is made by reception and transmission of radio signals between a master telephone set connected to a telephone line and a portable subsidiary telephone set, there is a problem of stealing of the line such that the master telephone set is enabled by a subsidiary telephone set belonging to a third party to make telephone communication without permission. Therefore, a pilot signal for enabling the master telephone set of each of radiotelephones is made to have its own specific frequency. However, since such a pilot signal is continuously transmitted during communication for holding of the line, the frequency of the pilot signal cannot be selected from those within an audio frequency band. Further, the upper limit of the frequencies of the pilot signals out of the audio frequency range is restricted in consideration of modulation of the pilot signals. The range of the frequencies as employed for the pilot signals of the radiotelephony is thus limited, leading to inevitable approximation of the frequencies of the pilot signals. Accordingly, in a conventional radiotelephone, there is such a disadvantage that stealing of the line cannot be completely prevented. Although it has been proposed to encode of the pilot signals for effectively preventing stealing of the line, additional components such as encoders and decoders are required in this case, leading to a costly radiotelephone.